Tuberculosis (“TB”, short for tubercle bacillus, also referred to as “MTB”), is caused by Mycobacterium infection, most typically, Mycobacterium tuberculosis, and ranks in the top three etiologies of infectious disease mortality. Drug resistant strains make this pathogen especially dangerous as both a general health hazard and a potential bioterrorism agent. Tuberculosis primarily impacts the lungs, although it can also affect other parts of the body, such as the brain, the kidneys, or the spine, and it is spread by transmission of respiratory fluids through the air (e.g., as a result of coughing or sneezing by an infected person). While many TB infections are latent (asymptomatic), approximately 10% of these latent infections eventually become active disease which, when untreated, kills more than 50% of infected patients.
Approximately one third of the world's population is thought to have been infected with M. tuberculosis. Although the absolute number of TB cases has been decreasing since the early-mid 2000's, according to the Centers for Disease Control (CDC), nearly 9 million people around the world became sick with TB disease in 2011, and there were around 1.4 million TB-related deaths worldwide. Moreover, TB is a leading killer of people who are TB infected. Also according to the CDC, a total of 10,528 TB cases (a rate of 3.4 cases per 100,000 persons) were reported in the United States in 2011. Therefore, there remains a need worldwide for preventative and therapeutic treatments for TB.
The treatment of TB is difficult and requires administration of multiple antibiotics over a long period of time (e.g., 6-9 months). Accordingly, patient compliance with the completion of treatment can be challenging. Moreover, antibiotic resistance is a problem, where multiple drug-resistant tuberculosis (MDR-TB) infections (caused by an organism that is resistant to at least isoniazid and rifampin, currently the two most potent TB drugs) continue to increase in prevalence. In addition, although rare, cases are also emerging of extensively drug-resistant TB (XDR TB), a type of MDR TB that is resistant to isoniazid and rifampin, plus any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin).
Prevention of tuberculosis in the United States is addressed primarily through TB screening and monitoring programs and in other regions of the world, is addressed by vaccination with the Bacillus Calmette-Guérin (BCG) vaccine. The BCG vaccine is a live vaccine originally derived from a strain of Mycobacterium bovis that was attenuated by Calmette and Guerin at the Pasteur Institute in Lille, France in the early 1900's. While the BCG vaccine showed some effect in certain pediatric cases, the vaccine has not been shown to be effective in adult TB, its efficacy wanes in teenagers and young adults, and the vaccine can interfere with tuberculin skin tests, which is a primary means of detection of TB used in screening methods of prevention.
It is known that CD4+ Th1 cells target intracellular pathogens such as M. tuberculosis via the production of interferon (IFN) gamma, and that the generation of CD8+ cytotoxic T cells (CTLs) is critical in combating TB (Billeskov et al, J. Immunol. 179:3973-81 (2007); Elvang et al, PLoS One 4:5139 (2009)). However, CD4+ Th1 and CD8+ T cells alone do not explain the resistance/susceptibility to TB as might be expected from a disease that also manifests itself extracellularly. It is now apparent that extracellular targeting of TB falls within the purview of a CD4+ T cell subset named Th17 for the cytokine it secretes, IL-17 (Hunter et al., Nat Rev Immunol 5, 521-31 (2005)). The magnitude of the Th17 response has been associated with clinical outcome (Khader et al., Nat Immunol 8:369-77 (2007)) and disease severity in TB (Chen et al, J. Clin. Immunol. published online 2010). If both Th1 and Th17 responses are required for productive immunity (Khader et al., supra (2007); Khader and Cooper, Cytokine 41:79-83 (2008)), TB may successfully escape from the immune system by thwarting either pathway. Indeed, it has been reported that TB can lead to downregulation of the IL-6 receptor on T cells (Chen et al., supra, 2010), IL-6 being a key cytokine in the induction of Th17 cells, thus negating the Th17 pathway and subverting an important component of the immune armamentarium. Finally, recent studies by Ordway et al. (Clin Vaccine Immunol. 2011 September; 18(9):1527-35, Epub 2011 Jul. 27) showed that BCG vaccination significantly delayed but did not prevent the emergence of the Treg population of CD4+ T cells, further diminishing long-term protection. This paper compared the host immune responses to two highly virulent W-Beijing strains in control mice and in mice previously vaccinated with the Bacillus Calmette-Guérin (BCG) TB vaccine. Results showed that the CD4+ effector T cell response peaks at around one month post-challenge, then wanes by day 60 concomitant with an increase in the frequency of Treg cells (CD4+Foxp3+ cells) in the lungs, draining lymph nodes, and spleen.
An effective post-exposure vaccine has been an ambition of many researchers in the field for years. Such vaccines have not, however, been widely successful or have not yet been proven efficacious in humans. For example, certain vaccines evaluated by scientists at Colorado State University that focused on antigens found in short-term culture filtrates, e.g., a subunit vaccine consisting of mid-log-phase culture filtrate proteins (CFP) from M. tuberculosis emulsified in MPL-TeoA adjuvant and supplemented with recombinant interleukin 2 (rIL-2) or a DNA vaccine encoding antigen 85A (Turner et al., supra 2000), were not successful in modulating the course of an aerosol infection with M. tuberculosis. In 2010, Bertholet et al., reported a recombinant subunit vaccine for TB, called ID93, which combines four antigens belonging to families of Mtb proteins associated with virulence (Rv2608, Rv3619, Rv3620) or latency (Rv1813) in a stable oil in water monophosphoryl lipid A-based adjuvant. In three animal models for TB, the vaccine was immunogenic and showed protection from challenge with virulent TB strains in two of the models. However, recombinant proteins are generally not immunogenic alone, and adjuvant development for the successful use of recombinant proteins such as ID93 in humans remains a challenge. While such adjuvants are the subject of ongoing investigation and clinical trials, it remains to be seen whether a subunit vaccine such as ID93 will be efficacious in humans as a pre- or post-exposure vaccine. Several other vaccine candidates are in development, but none have yet shown efficacy in a human clinical trial. In February 2013, the most advanced new vaccine against TB, a recombinant modified vaccinia virus Ankara expressing antigen 85A and developed by researchers at Oxford University, was reported to be unable to confer significant protection against tuberculosis or M. tuberculosis infection in infants (Tameris et al., Lancet. 2013 Mar. 23; 381(9871):1021-8).
Part of the challenge in developing a pre- or post-exposure vaccine lies in antigen selection. An exhaustive study by investigators at Colorado State University (CSU) in 2000 (Turner et al., supra 2000) indicated that there was no apparent material benefit from post-exposure vaccines based on certain “immunodominant” antigens of M. tuberculosis in the therapeutic setting. A review of antigens and current vaccines in development can be found in Orme et al. (Drugs (2013) 73:1015-1024). While immunodominant antigens still remain the focus of many TB vaccine efforts, investigators at CSU have recently also focused on antigens made by the bacillus under stress conditions (e.g., necrosis, host nitric oxide production, etc); these include what are regarded by some as “latency” antigens. Included in such latency antigens are antigens involved in a specific action by bacilli in primary lung granulomas to produce proteins for the purpose of accumulating iron. The CSU investigators have extensively defined the pathology of the disease process in guinea pigs infected by low dose aerosol with M. tuberculosis. One study, in which iron distribution in the primary lesion (as part of the dystrophic calcification process) was a primary focus, produced the surprising observation that both extracellular bacilli in the necrotic center, as well as bacilli probably still in macrophages on the rim of this structure (Lenaerts et al, 2007; Basaraba et al, 2008; Orme et al, 2011), were surrounded by ferric iron. These investigators had noticed previously that mice chronically infected with TB had T cells in the lungs that recognized bacterial ferritin and produced IFN-γ. A subsequent fusion protein vaccine was made that included this antigen, and it had modest protective activity in the guinea pig post-exposure model. The CSU investigators report a recombinant subunit vaccine formed from “iron accumulation” proteins (also referred to herein as “iron metabolism antigens”), namely Rv1909, Rv2359, and Rv2711 (Orme, Drugs (2013) 73:1015-1024).
Finally, as discussed in detail in Orme et al., 2011, supra, the prevailing view of the pathogenesis of TB, which has been in place for many years, is that inflammation drives cellular recruitment and granuloma formation, and the granulomas become neovascularized. The center of the lesion develops necrosis, while lymphocytes are held at the periphery of the lesions. These lesions can then cavitate, releasing bacilli into the airways. However, more current, alternative models of TB infection supported by more recent data, point to the primary lesion and its necrosis being not an end point, but rather a very early event, independent of the generation of acquired immunity. In this model, collapse of local vasculature allow bacilli to persist and survive, perhaps in primitive biofilms, secondary lesions form in the lungs, T cell immunity drops off despite survival of bacilli, and some of these lesions erode into adjacent airways or blood vessels, allowing the dissemination and escape of the TB infection. These differences in the view of pathogenesis of TB relate directly to how a prophylactic or therapeutic vaccine should be designed, again leading back to the idea that targeting immunodominant antigens based on primary lesions has not resulted in an efficacious vaccine to date. Indeed, a lack of understanding of the biology of the tuberculosis pathogen has been a major obstacle in vaccine development.
Therefore, there remains a need in the art for an improved pre- and/or post-exposure vaccine for TB.